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Understanding ciliated epithelia: the power of Xenopus.

Abstract
Ciliated epithelia are important in a wide variety of biological contexts where they generate directed fluid flow. Here we address the fundamental advances in understanding ciliated epithelia that have been achieved using Xenopus as a model system. Xenopus embryos are covered with a ciliated epithelium that propels fluid unidirectionally across their surface. The external nature of this tissue, coupled with the molecular tools available in Xenopus and the ease of microscopic analysis on intact animals has thrust Xenopus to the forefront of ciliated epithelia biology. We discuss advances in understanding the molecular regulators of ciliated epithelia cell fate as well as basic aspects of ciliated epithelia cell biology including ciliogenesis and cell polarity.

Figure 2. The differentiation of Xenopus ciliated epithelium. A: Ciliated cell and ionocyte precursors reside in a sublayer of the epidermis at Stage 17 and undergo radial intercalation to join the outer epidermis by Stage 25. B: Regulation of cell fate in the skin of Xenopus. Inhibition of the Notch pathway leads to an increase in ciliated cell and ionocyte differentiation. Activation of the Notch pathway, by silencing miR449 for example, leads to a severe decrease in ciliated cells. Ectopic expression of FoxJ1 leads to outer epithelial cells that are capable of generating 1-2 motile cilia. Ectopic expression of FoxI1 leads to an increase in ionocytes. Reproduced with permission of the Publisher, John Wiley & Sons

Figure 3. Steps of ciliogenesis in multiciliated cells. A simplified diagram of ciliogenesis that includes centriole duplication (1), fusion of basal bodies to apically migrating vesicles (2), docking with the apical membrane (3), targeting of cilia components to the basal body and ciliary membrane (4), and axoneme elongation (5). Reproduced with permission of the Publisher, John Wiley & Sons.

Figure 4. The planar polarity of motile cilia in multiciliated cells. A: Diagram of the non-cell autonomous effect of the PCP pathway. Wild type cells direct their cilia in the posterior direction (left, arrowheads represent cell polarity). Wild type ciliated cells abutting mutant clones are instructively oriented towards low levels of Vangl2, or high levels of Fz-3 (middle) and away from high levels of Vangl2 (right). B: Externally applied fluid flow is sufficient to orient motile cilia. C: Model for two step process driving cilia polarity. PCP signaling cues weakly bias the orientation of motile cilia, thus initiating a positive feedback loop in which cilia both generate flow as well as reorient in response to the prevailing flow (Basal bodies are red and rootlets are green). Reproduced with permission of the Publisher, John Wiley & Sons.